Multichip modules (MCMs) for electronic interconnections are classified into three types MCM-L, MCM-D and MCM-C. MCM-Ls are produced on organic substrates such as copper clad FR-4, epoxy glass, by multilayer laminate processes. MCM-Ls have the advantages of being available in large volume production at relatively low cost. The disadvantages of MCM-L are moisture sensitivity, low conductor routing density, poor thermal conductivity through the substrate, a high coefficient of thermal expansion compared to the integrated circuits to be mounted on them, and MCM-L is not suitable for a hermetically sealed package.
MCM-D is thin film deposited multichip module on a polished substrate of a ceramic or silicon. Metallization patterns are etched using photolithographic techniques, insulating layers are organic polymers, polyimide or benzenecyclobutane polymers. The vias in the insulating layers are also formed by photolithographic techniques. Manufacturing costs for MCM-D are very high, and have hindered wide use of the technology. The organic insulating layers of the MCM-D limit the temperature range in MCM-D manufacture and use, make the electrical properties sensitive to moisture, and prevent its use in hermetically sealed packages.
MCM-C is ceramic multichip module. MCM-Cs are usually multilayer conductive patterns joined together in a monolithic structure by cofiring. Each layer is formed from a green ceramic tape. The required via pattern is punched or laser drilled in the green tape. The conductive pattern for each layer is printed green tape, and the green tape layers then laminated together and cofired. The major problem of cofired MCM-C is the variation in size due to shrinkage in firing. Large scale electronic equipment manufacturers have successfully compensated for this problem for in-house production of long running part numbers, but shrinkage variation continues to plague the manufacture of MCM-C in small to medium scale production.
The transfer tape modification of the MCM-C process combines standard thick film hybrid circuit techniques with cofired green tape. The substrate is a fired ceramic. A hybrid thick film conductive pattern is printed on the substrate and fired. The required vias are made in the green tape by a laser before laminating the green tape to the fired ceramic substrate. The via diameter can be controlled much better than the via diameter in a conventional screened thick film dielectric layer, but there is a tendency for laser drilled green tape layers to form cracks around the vias.
The minimum via diameter that could be reliably printed and fired with screen printed dielectrics is 0.3 mm (12 mils). Vias printed with smaller diameter vias frequently close up when the screen printed dielectric layer is fired. So there was no method available to use a conventional thick film dielectric and get small diameter vias, i.e. diameters of 0.15 mm (6 mils) down to 25 .mu.m (1 mil).
In the prior art of hybrid circuits, lasers have been used to form holes through both fired ceramic substrates and through individual green ceramic layers before firing. There is no teaching in the prior art either of hybrid circuits or of MCM-C of a method of manufacturing multilayer circuits wherein vias between previously fired layers are formed by controlling the applied laser energy in proportion to the thickness of the dielectric so that via penetrates through the dielectric to contact the desired metallization layer without penetrating through the metallization layer.
Lasers are used for trimming thick film resistors through a glass seal layer. The cuts in the resistor are made through the glass. The glass of the seal layer does not contain any ceramic component, it is at least partially transparent to the laser, and the glass melts and reflows over the edges of the laser cut so no opening remains from the resistor layer through the glass.